ABSTRACT Driving and walking are important aspects of everyday life, yet car crashes and falls pose serious public health concerns. Distraction and multi-tasking while driving or walking often lead to disastrous results, indicating the importance of attention and cognition for safe mobility in the community. Parkinson disease (PD) affects over 1 million Americans at an annual cost of $25 billion, and specifically impacts attention and the ability to dual- task. Drivers with PD are at particular risk for driving cessation or crashes due to progressive difficulties in motor function and cognition that can lead to loss of independence, depression, isolation, and less participation in the community. A characteristic feature of cognitive impairment in PD is difficulty to begin or change self- initiated movement based on implicit cues. People with PD rely more heavily on external cues to overcome their slowed and small movements. For example, stepping to the beat of a metronome or stepping to lines on the floor improves walking. We will test the hypothesis that PD reduces the ability to flexibly allocate attention between arm and foot tasks based on accuracy constraints. Aim 1: Compare the effects of explicit instruction of task prioritization with implicit accuracy demands on shifts in attention during concurrent reaching and foot- pedal tracking tasks in participants with PD and age-matched healthy old adults. Aim 2: Compare the effects of explicit instruction of task prioritization with implicit accuracy demands on shifts in attention during lane-keeping (steering) and car-following (braking/accelerating) behavior during simulated driving performance in drivers with PD and in age-matched healthy old adults. Aim 3: Identify the effect of dopaminergic medication state on attention switching to explicit and implicit cues during driving-like tasks and simulated driving performance behavior in adults with PD. The long-term goal of this research is to characterize impairments in dual task abilities for driving and develop and implement intervention programs to reduce driving difficulties for people with PD.